The general concept of casting thin metallic sheet, strip, foil or ribbon relies on the use of a rapidly rotating substrate, such as a roll or belt that is cooled, and a source of molten metal which is solidified on the substrate in a manner which produces acceptable quality. The substrate must be properly cooled to extract the heat from the molten metal and cause the melt to rapidly solidify.
One of the most difficult problems associated with direct strip casting is the control of gage across the width of the strip. To permit the final product to meet commercial requirements, the variations in thickness across the strip width must be accurately controlled. The quality of the surfaces of the strip must also be controlled to avoid cracks, tears, folds or scale. The cast strip must also control the solidification to be uniform and avoid internal shrinkage voids or cracks.
Melt drag process is normally considered to be directed to casting thicker strip, typically above about 0.01 inches (about 0.25 mm). The molten metal is dragged from a nozzle positioned close to a rotating substrate. U.S. Pat. Nos. 3,522,836 and 3,605,863 use a convex meniscus of molten metal below a nozzle which is contacted by a rotating substrate to draw material from the meniscus. The heat extracting substrate, such as a water cooled drum, moves in a substantially parallel path to the outlet orifice of the nozzle.
In the melt drag process, molten metal forms a meniscus held on by surface tension at the outlet of the casting nozzle. The meniscus is then dragged onto the rotating drum or belt which is continuously cooled. However the melt drag process is severely limited in production speed due to the nature of the meniscus stability and melt flow restrictions. The lower line speeds used are restrictive, particularly to amorphous strip production which require very rapid quenching. U.S. Pat. No. 4,479,528 is typical of nozzles used for casting at a position below the top of the roll.
Planar flow casting systems are generally considered for casting thinner gage materials. Existing strip casting nozzles used for planar flow casting require different features than for planar drag casting. In planar flow, nozzles such as taught in U.S. Pat. No. 4,771,820 and U.S. Pat. No. 4,142,571 have molten metal which falls generally perpendicular to the top of the rotating substrate. The flow of molten material through a slot in the nozzle depends generally on the dimensions of the slot opening, the shape of the nozzle lips, the distances between the lips of the nozzle and the rotating substrate, the head pressure of the melt and the rotation speed of the substrate. In planar flow casting systems, the level of molten metal on the rotating substrate has always been below the molten metal bath level in the pouring box or supply vessel.
In the continuous production of narrow strip, the use of jet casting has been used which directs molten metal under pressure onto the top of a rotating roll. This process has a width limitation due to the difficulty in controlling the jet uniformly even for very short distances. It has been extremely difficult to match a plurality of jets with a uniform spacing and velocity which would provide a uniform pool at the surface of the substrate. Typically, the jet interactions cause ridges between pools and do not apply a uniform thickness across the width of the strip.
The use of two rotating rolls to continuously cast strip has also been attempted with limited success. U.S. Pat. No. 3,862,658 discloses a system for producing amorphous strip using two counter-rotating rolls.
Another strip casting system is called melt overflow which is characterized by the rotating substrate forming the horizontal end wall containment of the molten metal bath. U.S. Pat. Nos. 4,813,472 and 4,819,712 are typical of this approach where the molten pool on the substrate is at about the same elevation as the molten metal in the pouring box.
The progress made in strip casting has resulted in many refinements in the understanding of the basic interrelationships and variables required for uniform strip casting. Numerous modifications and innovations have been developed relating to tundish design, nozzle construction and substrate technology. The various nozzle dimensions evaluated for commercial production have been inadequate to produce the desired uniform strip. The critical dimensional relationships between the casting nozzle and the rotating substrate have yet to be defined which are capable to produce the uniformity and ranges of strip widths and thickness required.
In the past, planar flow casting has balanced the flow of molten material onto the substrate to equal the amount of material required by the pulling action of the substrate. The amount of material which can be in contact with the rotating substrate and solidified in a controlled manner has been limited in the past. The molten material could be pressurized only to a level which did not exceed leakage between the nozzle and substrate. Adjustments in rotation speeds of the substrate were limited to the strip thickness being cast and the cooling capabilities of the substrate. Substrate cooling will control strip thickness in combination with the amount of time the substrate is in contact with the molten pool. However, the cooling may also contribute to freezing of the molten metal in the area of the nozzle discharge. Long contact time will also require a longer contact distance along the arc of the substrate which previously required greater head pressures in the supply of molten metal. These conditions require improved nozzle lip strength to withstand the pressures or a reduction in production speeds if the thickness is to be adjusted and positive seals maintained within the nozzle. Slower wheel speed will also contribute to more freezing in the nozzle. Thicker strip will also have more heat which needs to be removed and complicates the cooling requirements for controlled solidification.
Another problem associated with prior planar flow casting systems was the gap distances between the casting apparatus and substrate being very small and requiring constant attention. This included measuring systems to constantly monitor the gap distances and numerous means to prevent or remove build-up of molten metal on the substrate. Serious restrictions on the static melt pressure tolerated were due to the very small gaps being used.
Accordingly, a new method and apparatus for casting thin metallic or amorphous strip is needed which overcomes the disadvantages of the prior art structures. The desired system must have an improved flexibility which leads to a more uniform cast product and which can produce a broader range of strip widths and gages. A new casting system is also needed which extends the tolerable gap dimensions and static pressures for casting uniform strip.